[78-L] World 1920s Records

[Jamie Kelly]

The terminology you and many computer programs and pieces of equipment use
sometimes really confuses the situation because they are sometimes used
incorrectly and I can't tell if you are or are not.  What Jamie is
discussing are the records which run at constant linear velocity, which
means that the groove speed is constant while the rotational speed changes
as the reproducer location moves inward or outward.  The rotation is faster
at the center than at the rim.  What he means by "co-ordinates formula" is
the ratio of the starting and ending rotational speeds so that he can
restore the recording which is being played at constant rotational velocity
to its original constant linear velocity status.

In analog the pitch and tempo remain in coordination with each other. 
Raise the tempo and you raise the pitch.  Slow down the tempo and you lower
the pitch.  THIS IS NOT PITCH SHIFTING, although many turntables and
computer programs call it by this name because they are designed by IDIOTS,
or Computer Geeks.  In digital (as well as some advanced analog devices such
as the Whirling Dervish or the Eventide) you can change the pitch while
keeping the tempo constant, or vary the tempo while keeping the pitch
constant.  THIS IS PITCH SHIFTING, and is not what Jamie needs.  

Jeff, what do you and Sound Forge mean?  From the daze of numbers below it
sounds like you are shifting the pitch, and then making an adjustment to the
mechanical rotational speed of the record.  That louses things up.  What
Jamie needs is some way to play the record at constant rotational speed and
vary the pitch and tempo TOGETHER in the computer. 
 

Mike Biel  mbiel at mbiel.com 

Yes quite true should be referred to as speed and not pitch.

I should of mensioned, I have recorded the disc at 33/3 rpm.

Jamie

Jamie


> There's a simple formula to convert a frequency ratio to cents. 
> Suppose you have a signal of 430 Hz, and you want to speed up the 
> recording to where the signal is 440 Hz (concert A). The formula is:
> cents = 1200 * (log(desired frequency / current frequency) / log(2)) 
> In this case, it would be:
> cents = 1200 * (log(440 / 430) / log(2)) The base of the logarithm 
> doesn't matter as long as it's the same in both the numerator and 
> denominator. Using the natural log, we get:
> cents = 1200 * (0.022989518224698739423565307535635 /
> 0.69314718055994530941723212145818) cents = 1200 * 
> 0.033166863935199317315221406257726 cents = 
> 39.800236722239180778265687509271 Rounding to one place after the 
> decimal point, we see that we need to speed up the recording by 39.8 
> cents. - Jeff Lichtman

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